Charging of Solutes and Interfaces in Nonpolar Liquids

Dennis C. Prieve and Paul J. Sides, Chemical Engineering

The mechanisms by which solutes and interfaces become charged are studied for nonpolar liquids like dodecane when doped with charge agents like poly(isobutylene) succinimide. Mechanisms for charging of solutes are suggested by observations of how the number density of charges and their mobility depend on total concentration of charge agent. Conductivity measurements alone are not sufficient to decouple the contributions from number density and mobility of the charge agent. Because the Debye length depends only on the solution’s ionic strength and permittivity (the latter is usually known), determination of the Debye length (e.g. from measurements of doublelayer force between two microscopic particles or between a particle and a plate) provides a direct determination of the concentration of charge carriers (to the extent that charge carriers can be considered univalent). However, they can be inferred using Debye length from measurements of capacitance, which are much quicker and easier to perform than force measurements, although the latter is also used to verify Debye lengths inferred from the former in a few cases. Once the concentration of charge carriers is established, the mobility and size of the charge carriers are inferred from conductivity. Size is confirmed using dynamic light scattering.

Mechanisms of charging of interfaces is usually inferred from observations of the zeta potential of the interface as a function of concentration. Here such measurements are made on flat interfaces with nonpolar fluids using the ZetaSpin device developed under a previous NSF grant. One important advantage of this device is that it can also monitor the dynamics of charging, which provides additional clues as to the mechanism of charging.

Intellectual Merit
Aqueous electrolyte solutions have been studied extensively for over a century, possibly starting with such well known scientists as Arrhenius (1887), Bjerrum (1918), Debye & Huckel (1923), Onsager (1927) and Fuoss (1934). Mechanisms for charging of ions by dissociation of ion pairs is firmly established. By contrast, very little is known about charging mechanisms in nonpolar media. The reason is that their existence and importance was not recognized until fairly recently. At the risk of being accused of hyperbole, we feel we are exploring a strange new world where many of the theories we have learned for aqueous solutions do not hold.

Broader Impacts
Understanding these charging mechanisms is important to the development of new technologies such as E-Ink’s highly flexible electronic display, which was recently incorporated in Sony’s E-book. The extremely low conductivity of nonpolar liquids means that such displays can be maintained with very low power consumption compared to liquid-crystal displays. While E-Ink’s technology for black-and-white displays is already highly developed, further research and development is still needed to extend this technology to color displays.

Besides providing research training for one or two PhD students and a research experience for a few undergraduates, new experiments are being developed to be included in a course on advanced experimental techniques being offered on a continuing basis to advanced undergraduates and graduate students (including parttime MS students from local industry) in our Colloids, Polymers & Surfaces program. A small business (ZetaMetrix) was formed to market the ZetaSpin device.